Laser-plasma resonance condition in the middle-infrared spectral range*

The resonance-absorption condition in the laser-plasma interactions has been considered to follow the wavelength dependence of the critical plasma density. We experimentally demonstrate that this assumption fails in the middle-infrared spectral range for nano-plasma targets, while it is valid for visible and near-infrared wavelengths. The experiment is enabled by extending the pump-probe technique to a scheme with a probe pulse tunable in a broad spectral range from 0.6 μm to 2 μm wavelength. A thorough analysis supported by molecular dynamic (MD) simulations indicate that the observed transition in the resonance condition is caused by the reduction of the electron scattering rate and the associated increase of the cluster outer-ionization contribution. A new expression for the nano-plasma resonance density is derived based on experimental results and MD-simulations. The findings are important for a broad range of plasma experiments and applications, since the extension of the laser-plasma interaction studies to longer wavelengths has become increasingly topical.